Area Forecast Discussion
Issued by NWS Great Falls, MT
Issued by NWS Great Falls, MT
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706 FXUS65 KTFX 091554 AFDTFX Area Forecast Discussion National Weather Service Great Falls MT 954 AM MDT Sun Jun 9 2024 .SYNOPSIS... Showers and thunderstorms increase over Southwest Montana this afternoon before spreading northward into central and north- central areas later in the evening and the overnight hours. The stronger storms may contain strong, gusty winds, hail, and heavy downpours, particularly for southwestern locations. Shower and thunderstorm activity diminishes Monday with breezy and much drier conditions expected heading into the mid-week period. && .Update... Morning update has been published, with only minor adjustment made to the on-going forecast. Main adjustment was to increase PoPs an additional 10-20% across the CWA through tonight as latest Hi-Res guidance continues to indicate numerous showers and thunderstorms lifting east and northeast across Southwest through North Central Montana through the overnight hours. Early morning convection across Southwest Montana will predominately remain near the Idaho border through this morning, with additional showers and thunderstorms developing between 10am-2pm over the higher terrain of Northeast Idaho/Southwest Montana and then lifting northeast over lower elevations of Southwest and into Central Montana through 6pm. Additional showers and thunderstorms will then overspread the remainder of Central and North Central from west to east from the late evening through morning hours on Monday as the shortwave moves overhead. A Marginal Risk for severe thunderstorms continues southeast of a general Dillon, to Three Forks, to WSS, to Lewistown line, with damaging wind gusts being the primary concern. Additionally, bursts of heavier rain over the higher terrain where an appreciable snowpack remains could lead to rises on creeks and streams emanating from the mountains. - Moldan && .AVIATION... 09/12z TAF Period VFR conditions continue through the beginning of the TAF period with conditions having potential to degrade across Southwest Montana late this afternoon through the evening hours tonight. Mountain obstruction is expected across the Southwest mountain ranges this evening and potentially into Central Montana tomorrow morning. Thunderstorms are likely to develop between the 09/20z and 10/06z period for Southwest Montana airfields, but quickly move through with some lingering rain showers behind the storms. Primary concerns for any developed thunderstorm is lightning, gusty winds, small sized hail, and a brief heavy downpour that can lower visibility. Scattered rain showers move into Central Montana after the 10/00z time frame into tomorrow morning. Webb Refer to weather.gov/zlc for more detailed regional aviation weather and hazard information. && .PREV DISCUSSION... /ISSUED 527 AM MDT Sun Jun 9 2024/ Lower level north/easterly flow has persisted through the overnight hours for continued breezy conditions and scattered to broken clouds, mostly over central and north-central MT. Lower level cloud cover associated with this set up is expected to retreat to the northeast as a southwesterly flow aloft begins to develop ahead of a shortwave currently moving into the Pacific Northwest. An initial wave of weakening showers and isolated thunderstorms moving through Central Idaho is already being tracked on radar and should reach Beaverhead County later this morning in the form of isolated lighter end showers/virga and perhaps a lightning strike or two. Southwesterly flow aloft becomes more established ahead of the approaching shortwave later today, providing ascent, shear, and modest instability for afternoon and evening shower and thunderstorm development. The highest CAPE (500 to 1,000 J/kg) looks to be for areas south of I90, where gusty winds and hail may accompany the stronger cells. Precipitable water values will generally range in the 0.75 to 1 inch mark, so localized heavy downpours can also be expected. This activity will lift northward into Central/North- central MT during the evening and the overnight hours, but the loss of diurnal heating will result in a more benign brand of showers and general rains. While hi-res guidance does bring a secondary round of showers and storms to the southwest late tonight, a more stabilized atmosphere should preclude the potential for stronger development in a similar fashion as northern areas. Westerly winds increase and things dry out on Monday following the passage of the shortwave, but another weak perturbation does move across the region during the afternoon hours for additional isolated to scattered showers and thunderstorms, mostly along the Hi-Line and over and near areas of higher terrain elsewhere. Ensembles remain in fairly good agreement with a zonal flow aloft settling in for the mid-week period. H700 to H500 flow will be on the stronger side for this time of time of year, mostly between 40 and 50 kts or 2 to 3 standard deviations above climatology according to the NAEFS (North American Ensemble Forecast System). Deep layer mixing should encourage breezy to windy conditions, especially over the plains and the Rocky Mountain Front on Wednesday. Wind exceedence probabilities suggest that peak wind gusts will mostly fall between the 35 and 55 mph range except along the Rocky Mountain Front and adjacent high plains where gusts up to and exceeding 60 mph will be more common. The primary impact will be to summer outdoor recreation and travel; however, afternoon temperatures in the 70s and 80s combined with the said winds and minimum relative humidity values in the teens and 20s is expected to accelerate the curing/drying of grassland fuels. While no fire products are anticipated this time around, this may set up fire weather concerns in the future. Uncertainty remains in terms of forecast details for the upcoming weekend, but ensembles continue to highlight a Pacific trough exerting an influence over the Northern Rockies. At this time, there is an expectation for breezy conditions, isolate to scattered shower and thunderstorm activity, and a slight cool down for Saturday and Sunday. - RCG && .PRELIMINARY POINT TEMPS/POPS... GTF 74 56 80 50 / 10 70 30 10 CTB 69 51 77 49 / 10 60 30 0 HLN 79 59 84 51 / 30 80 30 0 BZN 77 55 78 45 / 60 80 50 0 WYS 74 47 71 39 / 70 80 60 0 DLN 76 51 76 45 / 90 90 20 0 HVR 73 53 77 49 / 10 70 50 10 LWT 73 54 76 46 / 10 70 40 10 && .TFX WATCHES/WARNINGS/ADVISORIES... None. && $$ http://www.weather.gov/greatfalls